One category of motion sensors and fetal heart monitors uses piezoelectric ceramics which typically are mounted on a substrate disc. The substrate, in turn, is supported along its periphery. The supporting element or elements may be mounted in a housing or casing which then is engaged against the matter whose movement or vibration is to be sensed by an actuator member connecting the piezoceramic disc to the vibrating matter. Such movement or vibration induces flexing of the substrate and piezoelectric material, thereby inducing an electrical signal that can be processed to provide an output of the detected motion or vibration.
For example, the "Transducer 15135A Heart Sound" and "Transducer 13135B Heart Sound" devices available from Hewlett-Packard use a piezoelectric crystal transducer mounted on a substrate disc which, in turn, is supported along its peripheral margin. The annular support member for the substrate is carried by a housing that can be engaged tightly against the skin of a patient. Vibration is transmitted to the center of the disc by an actuator which makes contact with the mother's abdomen and the center of the piezo disc. If the magnitude of the vibrations is great enough, vibratory flexing of the substrate and piezoelectric crystal occurs. This results in an electrical signal that can be amplified and processed to detect vibrations within a frequency of interest, namely, vibrations induced by beating of a heart. Consequently, a noninvasive "heart sound" detector is provided.
U.S. Pat. No. 4,672,976 issued on Jun. 16, 1987 to Mark W. Kroll discloses a noninvasive heart sound sensor using an external hydrophone; U.S. Pat. No. 5,140,992 issued Aug. 25, 1992 to Allan J. Zuckerwar discloses a "passive" fetal monitoring system that uses piezoelectric polymer film; and the following patents disclose systems for processing signals from noninvasive heart monitors: U.S. Pat. No. 4,456,959 issued Jun. 26, 1984 to Toshinori Hirano et al., U.S. Pat. No. 5,524,631 issued Jun. 11, 1996 to Stephen A. Zahorian et al., and U.S. Pat. No. 5,209,237 issued May 11, 1993 to Felix Rosenthal.